The join is part of FROM. It runs first. WHERE, GROUP BY and HAVING then work on the joined rows, in the order you know from Chapter 7.

How many items does each order have? Join, then group:

SELECT o.id, COUNT(*) AS items
FROM orders o
JOIN order_items oi ON oi.order_id = o.id
GROUP BY o.id;

Count 0 with a left join

Count the employees in each department, Legal included:

SELECT d.name, COUNT(*) AS staff
FROM departments d
LEFT JOIN employees e ON e.department_id = d.id
GROUP BY d.id, d.name;

COUNT(*) counts rows, so Legal gets 1. That is wrong. Count a column of the right table that is never NULL in a real match:

SELECT d.name, COUNT(e.id) AS staff
FROM departments d
LEFT JOIN employees e ON e.department_id = d.id
GROUP BY d.id, d.name;

COUNT(e.id) skips NULL (Chapter 7), so Legal gets 0.